Unlike coherent (COH) detection, direct-detection (DD) does not straightforwardly support polarization multiplexing (POL-MUX) transmission of advanced modulation (ADV-MOD) formats (amplitude & phase or IQ). Multiple solutions have been proposed to nevertheless enable DD to support POL-MUX ADV-DD in order to attain substantially higher spectral efficiency (SE), which would be a laudable goal for applications wherein COH detection would be prohibitively expensive, such as in short-reach access and metro networks.
DD systems which detects signal phase in addition to its amplitude, akin to COH transmission, albeit without a local oscillator (LO) in the receiver (Rx), may be referred to as self-coherent (SCOH). There have been multiple approaches to attain SCOH [, . . . ].
One popular class of SCOH systems is based on remote (or self) heterodyne (HET), whereby the LO is effectively sent all the way from the transmitter (Tx) to the Rx, in the form of a pilot tone (p) offset by W from the transmitted data spectrum, where W is the bandwidth of the data (d). The spectral offset is required to accommodate the d×d term, whereas the desired COH-like p×d term in the photo-current is generated offset by W in the electrical domain. Such self-HET method has been proposed for DD OFDM transmission, but we mention here that it is also applicable to single-carrier rather than OFDM transmission. It may be used to detect ADV-MOD formats if the electrical signal is IQ demodulated. Unfortunately, just a single polarization may be effectively used for self-HET transmission due to a fundamental “remote LO-fading” limitation outlined below.